During my pregnancy, I've been fortunate in that I've not needed to invest in many pieces. I have a really kind co-worker who handed down a few pair of her maternity jeans - I've worn them daily. I purchased a pair from a brand called Angel Maternity jeans (also try this pair, or this pair of denim leggings) I've pretty much lived in them my entire pregnancy (even arguing an motion for summary judgment in a pair - shhh). As for dresses, I've mostly managed to stick with the same dresses I owned before; anything that has stretchy fabric is a 'go' (like this basic jersey dress by Leith).
There is this option from Bumpsuit which is a maternity wear brand. It's a great Fall layering piece.